Induction metal detectors are generally designed to transmit either continuous wave (CW) signals, so-called frequency-domain (FD) detectors or to use pulsed or rectangular signals, so-called time-domain (TD) detectors. For the purpose of this invention description, a transmit waveform is understood to mean coil current unless otherwise noted.
Often both types of designs use substantially similar receiver architectures: a preamp followed by one or more synchronous demodulation channels, integration and/or filtering, analog-to-digital conversion, and digital signal processing. To date, commercialized metal detectors that mix FD and TD in the same design are rare and tend to be user-selected to one mode or the other, but do not run simultaneously.
Time-domain detectors are often referred to as pulse induction (PI) detectors, as most designs create a short pulse of current using a switched coil. When the current is switched off, the result is a high voltage flyback. The decay of the flyback is usually critically damped with a damping resistor, and the decay of the flyback is monitored for perturbations due to nearby metal targets. See U.S. Pat. No. 5,414,411.
A typical pulse induction metal detector transmits a single pulse width duration of a consistent peak current amplitude, resulting in a single response that must be processed. Some methods have been described which use either multiple pulse width durations (see FIG. 1, U.S. Pat. No. 5,576,624) such as a series of short pulses 11 and long pulses 12. Some methods create differing peak current amplitudes (see FIG. 2 U.S. Pat. No. 6,653,838), such as a series of high current pulses 13 and a low current pulses 14. Either of the methods can produce variable responses to eddy current targets or to ferromagnetic ground or both. Typically such multiple responses are processed through multiple receive channels, whether such channels are realized in hardware, software, or a combination. These methods are analogous to so-called “multifrequency” metal detectors which use frequency-domain techniques.
PI detectors are often used in military and humanitarian demining. Some land mines include a magnetic trigger, so this application requires the use of bipolar pulsing to avoid the creation of a non-zero net magnetic field (see U.S. Pat. No. 6,653,838 and FIG. 3) which shows a series of positive pulses 15 and negative pulses 16. Additional benefits are possible with bipolar pulsing. Subtracting the responses of the two polarities substantially cancels induced signals from the Earth's magnetic field and other low-frequency interferers while maintaining eddy current induced target responses.
In many applications, a desirable feature in a metal detector is the ability to distinguish between various types of targets such as ferrous versus non-ferrous or low conductor versus high conductor. Currently available PI detectors generally exhibit poor discrimination capabilities. Frequency domain designs utilizing CW signals, especially sine waves, often use the target phase response to determine target characteristics. However, PI detectors generally achieve greater detection depths than do CW detectors, especially in ground which is high in mineralization or exhibits high magnetic viscosity. The ability to tune out mineralized ground is generally referred to as Ground Balance (GB). While both PI and CW designs include methods of ground balance, PI is inherently less sensitive to mineralization than CW. However, the GB method in many PI designs involves the subtraction of two signal samples, which not only reduces depth in general. The subtraction can also completely subtract out certain target responses, resulting in so-called “target holes” where particular targets cannot be detected at all.
Needs exist for improved metal detectors.